1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to an acetabular cup component of a total hip prosthesis.
2. Background Information
An implantable total hip prosthesis includes a femoral component and an acetabular cup component. The femoral component, typically made of a bio-compatible metal such as titanium, titanium alloy or cobalt chrome alloy, has a distal stem and a proximal spherical head. As used herein, the words proximal and distal are terms of reference that indicate a particular portion of a prosthesis component according to the relative disposition of the portion when the component is implanted. "Proximal" indicates that portion of a component nearest to the torso, whereas "distal" indicates that portion of the component farthest from the torso. The distal stem is configured to be received and fixed within the medullary canal of a femur. The femur is prepared to receive the distal stem by resecting the natural head and neck, and reaming the exposed proximal medullary canal. The proximal end of the femoral component has a neck and attached spherical head that, supported by the distal stem anchored in the medullary canal, extend from the resected proximal end of the femur to replace the natural neck and head of the proximal femur. The proximal spherical head may be integral with the neck and stem of the femoral component, or may be removably attachable to the neck by way of a locking taper connection, sometimes known as a Morse taper. The acetabular cup component is configured to be received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of the femoral component. The head articulates in the bearing cavity as a ball and socket to restore motion to a defective hip joint.
One known type of acetabular cup involves an acetabular shell made of a bio-compatible metal such as titanium or a titanium alloy, and a bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The acetabular shell is shaped generally as a hemispherical cup having a dome, or apex, at a proximal end and an annular rim at a distal end. Between the dome and rim, the acetabular shell comprises a shell wall defined by a generally convex proximal surface and a generally concave distal surface spaced from the proximal surface. The concave distal surface defines a shell cavity having an opening at the rim of the cup for receiving the bearing insert. The bearing insert has a generally convex proximal surface configured to be received and fixed within the acetabular shell in generally congruent engagement with the concave distal surface of the shell wall. The bearing insert also has a bearing cavity that opens distally for receiving the head of the femoral component. The bearing cavity is defined by a generally spherical concave bearing surface having a radius similar to that of the femoral head component. The concave bearing surface articulates against the surface of the spherical femoral head component.
Acetabular shells of the type described can be affixed to the acetabular bone by means of bone screws, bone cement, bone ingrowth or ongrowth, mechanical interference, or by a combination of two or more of these means. To promote bone ingrowth or ongrowth, the convex proximal surface of an acetabular shell, i.e., the bone-engaging surface, can be provided with a porous coating applied as sintered metal particles, spheres, or wire mesh, or as plasma sprayed metal, for example. In lieu of porous coating, the bone-engaging surface can be roughened by grit blasting. As an option, the porous or grit blasted surface can be coated with hydroxylapatite, applied by plasma spraying, to further enhance the osseoinductive characteristics of the acetabular shell. Fixation by mechanical interference can be accomplished by carefully reaming the acetabulum to a contour that, in combination with the contour of the proximal bone-engaging surface of the acetabular shell, results in a press-fit relationship between the acetabular shell and the acetabular bone. Also, gross mechanical interference between shell and bone can be accomplished by means of sharp protrusions on the bone-engaging surface of the acetabular shell, such as threads, grooves, fins, spikes, or other macro-textured surface features that penetrate into the acetabular bone as the shell is implanted.
Some acetabular shells that employ a press-fit relationship between the acetabular shell and the acetabular bone rely upon the acetabular shell being somewhat oversized in diameter relative to the diameter of the reamed acetabulum. Some such designs extend the oversized area well away from the rim of the cup, which can result in compressive forces being generated in the acetabular bone that have a significant axial component. This can lead to a risk of the acetabular cup being expelled from the acetabulum, incomplete seating and or fracture of the acetabulum. It would be desirable to provide an acetabular shell, designed to optimize a press-fit relationship to acetabular bone, that attempts to alleviate the forementioned risks. The present invention provides this and other desirable advantages.
Many acetabular shells that exploit gross mechanical interference with the bone to achieve fixation or stabilization can be somewhat difficult to orient accurately during implantation. This is because the protrusions that effect fixation must necessarily engage the bone before the main body of the acetabular shell engages the bone, resulting in the protrusions being committed to a particular path of penetration into the bone before the cup body has been assuredly positioned within the reamed acetabulum. It would be desirable to provide an acetabular shell, designed to employ mechanical fixation to bone, that can be oriented accurately in the reamed acetabulum before being driven into its fully seated position. The present invention provides this and other desirable advantages.